1. Field of the Invention
The present invention relates to a motor having a plurality of pairs of cores, each pair consisting of an inner core and an outer core, arranged next to one another along the axial direction so that the inner cores are in contact with each other, and a coil wound around each pair of cores.
2. Description of Related Art
A motor having a plurality of pairs of cores, each pair consisting of an inner core and an outer core, arranged next to each other along the axial direction so that the inner cores are in contact with each other, and a coil wound around each pair of cores, has been known for some time. For example, such a motor is described in Japanese Laid-Open Patent Application No. 10-271793. The motor described in this reference is a stepping motor, in which a lead screw section is provided on a motor rotor shaft to which a rotor magnet is fixed, and the motor rotor shaft and the lead screw section are rotatably supported with respect to a frame. A stator section of the motor consists of two pairs of cores. Each of the pairs has an inner core and an outer core. Each of the inner cores and the outer cores has teeth-like poles, and the inner core teeth-like poles and outer core teeth-like poles are alternately disposed and face the rotor magnet. Ring-shaped coils with coil bobbins are placed between the inner cores and the outer cores.
The stator section of the motor may be modified to simplify the structure of the stator section of this motor in a manner shown in FIGS. 3 (1) and 3 (2). FIG. 3 (1) is a side view of the motor with a partial cross section. To simplify the description, a rotor part with a rotor magnet in the motor is omitted. FIG. 3 (2) is a front view of the motor. The stator section includes a coil section 1, a case 2 provided to cover the coil section 1, and a frame 3 to which the coil section 1 and the case 2 are attached.
In FIG. 3, the coil section 1 has a ring-shaped coil 13 wound around a first pair of cores that includes an inner core 11 and an outer core 12. Each of the inner core 11 and the outer core 12 has teeth-like poles facing the rotor magnet of the rotor. The coil section 1 also has a ring-shaped coil 16 wound around a second pair of cores that includes an inner core 14 and an outer core 15. Each of the inner core 14 and the outer core 16 has teeth-like poles facing the rotor magnet of the rotor. The coil section 1 is structured in such a way as to have the core pairs arranged in the axial direction so that the inner cores 11 and 14 come in contact with each other.
The xe2x80x9cinner coresxe2x80x9d referred to herein are the cores that are disposed on the inner side, and the xe2x80x9couter coresxe2x80x9d are the cores that are positioned on the outer side along the axial direction, when the two pairs of cores are arranged in the axial direction, as the figure shows.
Connection terminals 17 are attached to the inner cores 11 and 14 to supply current to the coils 13 and 16. A rotor having a rotor shaft and a rotor magnet formed in a unitary structure is provided within the first pair of cores and the second pair of cores. However, the rotor is omitted from the drawings for simplicity of the illustration.
The coil section 1 having the coils 13 and 16 is covered by a cylindrical case 2. The cylindrical case 2 is slid over the coil section 1 in the axial direction and lightly pressure-fit over it, and the case 2 is connected with the first pair of cores and the second pair of cores. However, a part of the cylindrical case 2 is open to allow the connection terminals 17 to be exposed to the outside.
The motor described above may have the following problems. In the motor shown in FIG. 3, when the inner cores 11 and 14 and the outer cores 12 and 15 are joined by the case 2 made of a magnetic material, the coil 13 is expected to form a magnetic circuit surrounded by the inner core 11, the case 2 and the outer core 12, while the coil 16 is expected to form a magnetic circuit surrounded by the inner core 14, the case 2 and the outer core 15. In other words, the coils 13 and 16 must form independent phase A and phase B magnetic circuits. However, if the case 2 is installed by a light pressure-fit over the inner and outer cores, a small gap may be formed between the case 2 and the inner cores 11 and 14. In such a case, the phase A and phase B magnetic circuits on the coils 13 and 16 fail to fully become independent. Instead, a single magnetic circuit via the case 2 can be formed. This causes the magnetic balance between the phase A and phase B magnetic circuits to be unstable.
Furthermore, the stator section of the motor described above has a relatively complex structure. Also, although the motor shown in FIG. 3 has a relatively simplified structure compared to conventional motors, its angle precision cyclically fluctuates and its rotation may not be uniform when the motor is driven by exciting in phases 1-2. This may lead to poor detent torque and poor angle precision, which may in turn cause noise problems.
It is an object of the present invention to provide a motor that has a relatively simple structure and yet has a uniform rotational characteristic.
In accordance with an embodiment of the present invention, a motor has a stator with a simplified structure having a stator section including at least two core pairs. Each of the core pairs includes inner and outer cores. Coils are wound around the core pairs. A case made of a magnetic material covers the coils in a manner that the case more securely comes in contact with the inner cores and the outer cores. The case is welded to at least the inner cores to securely form independent magnetic circuits.
In accordance with another embodiment of the present invention, the case is welded to the inner cores and the outer cores to securely form independent magnetic circuits.
As a result, there is no need to incorporate steps in a manufacturing process, which could lead to increased costs, such as, for example, finishing a case with more stringent dimensional precision against inner cores or employing a deep drawing method in order to achieve a higher dimensional precision in the cylindrical curved surface of the case.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.